The present invention relates to a method and apparatus for converting the resolution of an image comprising lines of dots.
The invention addresses the problem faced, for example, by a facsimile machine having a printing unit that prints three hundred dots per inch, when facsimile data with a resolution of two hundred dots per inch are received. The conventional solution to this problem has been to convert the resolution by duplicating the dots in fixed rows and columns. For example, the horizontal resolution can be converted by printing the odd-numbered dots in each line twice and printing the even-numbered dots once, while the vertical resolution can be converted by printing odd-numbered lines twice and even-numbered lines once.
As will be illustrated later, however, this conventional scheme has the defect of distorting the gray scale of images in which different gray levels are represented by different proportions of black dots. Similar tonal distortions occur if the conventional scheme is used to convert the dot resolution of color images.